Size does matter for some

Ancient ostracodes, similar to these mussel shrimps, produced sperm longer than their body length (Source: Renate Matzke-Karasz)

Tiny mussel-like creatures living 100 million years ago made giant sperm longer than their own bodies, say scientists.

A study, published in the journal Science, reports that giant sperm still exist today in Drosophilia species - commonly known as fruit flies.

The insect is only a few millimetres in size, but can produce 6 centimetre - long coiled sperm.

A human sperm would have to be 40 metres long to measure up against a fruit fly's.

Up to now scientists had been unsure if such gigantism is a freakish one-off.

Giant sperm evolved long ago

Now the discovery that several extinct species of ostracodes, an ancient class of arthropods, displayed the same trait shows that making giant sperm is a long-standing and evolutionarily successful reproduction strategy.

"Giant sperm have been produced in at least some species over long periods of time, even though they come at an extremely high price for both males and females," says lead author Renate Matzke-Karasz of the Ludwig Maximilian University in Munich.

In most animals, including humans, reproductive success depends on males producing a large number of tiny spermatozoa, while females invest in a few large eggs.

But in some cases where sperm have to compete inside a female's body, the chance of successful fertilisation can be improved by increasing the size of the sperm cell.

Sperm preserved in fossils

Matzke-Karasz and colleagues studied the rarely preserved reproductive system of several ostracode fossils found in the Cretaceous Santana Formation of Brazil.

Using a new imaging technique known as 'holotomography' the team were able detect the organs, which are only 1millimetre long and used for transferring giant sperm.

The images showed several species of female ostracodes possessed two seminal receptacles at the end of long ducts. These could change size and shape after mating to accommodate the giant sperm.

Matzke-Karasz says the next step of their research will be to understand why and how giant sperm persisted for so long.